It is well known in the art that many oscillators have a stable (or non-oscillating) region of operation so that the oscillator circuits may not start up reliably. In other words, if the oscillator starts up in a random fashion, it may never oscillate. This explains why a start up circuit is normally required.
Another problem associated with the start up procedure for the oscillator is with the reference crystal. That is, the amplitude of the reference crystal attains a steady state after the power supply Vdd has reached a stable value. If the oscillator, which can be a voltage controlled oscillator (VCO) or a current controlled oscillator (ICO), tries to lock to the reference crystal during the transition period, it might enter the non-oscillating region of operation and thus never oscillates. This necessitates the provision of another start up circuit.
Various efforts have been made to guarantee the reliable start up of oscillator circuits. The start up circuit for an oscillator ensures that the oscillator always starts up in the proper region of operation during the power up procedure in order to regularly oscillate.
U.S. Pat. No. 5,534,826 issued to Shawn M. Logan provides a technique where the crystal connection to the crystal resonator is changed. This method guarantees the fast start up of the crystal oscillator. Fast start up of the reference crystal, however, does not guarantee the desired start up of a phase locked loop (PLL) oscillator (ICO/VCO).
One prior art technique is mentioned by Yasuo Tsuzuki, Takehiko Adachi and H Wen Zhang in an article entitled "Fast start-up crystal oscillator circuits," Proceedings of 49.sup.th Annual IEEE International Frequency Control Symposium, May 1995, pages 565-568. In this article, the gain of the oscillator circuit is increased at the start up stage by reducing the value of a user controlled impedance until the oscillation commences. This method, however, does not guarantee to take the oscillator out of its stable region if it is already in the stable region.
An alternative start up operation can also be established by using two circuits. The first stage consists of a Vdd detect circuit to detect the value of Vdd. The second stage consists of a circuit to detect the amplitude of the reference crystal. The trigger signal from the second stage enables the normal operation of the oscillator. Essentially, the oscillator is held in a stopped state until the reference crystal has reached the steady state. To detect a value of Vdd greater than Vdd/2 for all process corners and temperature, however, a large capacitor is required for the first stage. In addition, the amplitude detect stage for the reference crystal requires a large number of gates. Eventually, both these circuits occupy a large area.
It is therefore tried by the Applicant to deal with the above situations encountered in the prior art.